Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector CN9 from the PCB
Remove the connector from the DHW thermistor

Measurement:

Test 1.
Measure the resistance between pin 5 (Red) on connector
CN9 and the red wire on the DHW thermistor connector.
Test 2.
Measure the resistance between pin 6 (Black) on connector
CN9 and the black wire on the DHW thermistor connector.
Expect continuity / zero resistance on both tests.
Also inspect the condition of the harness, visually checking
for any damaged insulation.